Channel sealant compositions

ABSTRACT

A sealant composition comprising a non-crosslinked, elastomeric mastic and an extrusion-inhibiting amount of vulcanized rubber particles of irregular shape and having sharp edges and angular surfaces. When the composition is used as a channel sealant, the ability of the particles to deform at structural gaps while offering resistance to extrusion prevents the loss of the elastomeric mastic.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

FIELD OF THE INVENTION

This invention relates to sealant compositions which are particularlyuseful as channel sealants.

BACKGROUND OF THE INVENTION

Aircraft integral fuel tanks are so named because they are an integralpart of the aircraft frame and skin. While use of irregularly shapedcavities of wings and fuselage as fuel tanks is a necessary anddesirable utilization of space particularly in military aircraft, theproblem of sealing the cavities so as to inhibit hazardous fuel leakagehas been the subject of extensive research. Thus, the aircraft frame andskin must be sealed at every joint and fastener by a flexible materialto provide a permanent barrier to fuel leakage. Fuel resistantelastomers, such as polysulfides, fluorosilicones, polyesters,cyanosilicones, and urethanes have been used as sealants with varyingdegrees of success.

Integral fuel tank sealants can be divided into three types, namely, (1)filleting, (2) faying, and (3) channel. Filleting sealants are viscousliquid polymer mixes which are extruded over seams, into joint corners,over fasteners, and around openings for hydraulic lines and electricalconduits. Generally, this type cures to a tough rubber-like material ina few days at room temperature. Repair is accomplished by physicallyremoving the fillet and reapplying a fillet of the same or a compatiblematerial. In inaccessible areas it is difficult to make repairs. Fayingsealants are liquid elastomeric compounds, often thinned with a solvent,which are applied between skin and structural members. After applicationto the mating surfaces, the parts are fastened together. Most fayingsurface sealants are room temperature vulcanizing products that reachfull state of vulcanization in a few days. It is necessary todisassemble the skin and frame in order to repair faying surfacesealants. Channel sealants are non-vulcanizing mastics that are injectedinto grooves formed in the structure-skin joints. The sealants can beapplied as the aircraft is assembled or injected through ports afterassembly.

While any one of the three types of sealants described in the precedingparagraph can be used alone, to ensure reliability many aircraft oftenemploy two or all three. When the non-curing channel sealant is utilizedas the primary sealant for integral fuel tanks, a careful balancing ofproperties is required. The sealant must have an elastomericflexibility, it must adhere to metal with a tacky-type adhesion, it mustresist being pushed or extracted from the joint by fuel, and it mustretain these properties over a wide temperature range, e.g., from -65°F. to 350° F. The most common failure mode for channel sealants is gapextrusion caused by pressure build-up in the channel from thermalexpansion, fuel swell, internal tank pressures, and, to a lesser extent,joint flexing and gas formation from polymer degradation. In U.S. Pat.No. 3,580,870 a channel sealant composition is disclosed that includes asealant medium containing plastic balls or spheres graduated in sizefrom large balls to medium size balls to small balls. As described bythe patentee, the balls congregate or cluster along gaps between theskin sheet and wall of a fuel tank, thereby preventing escape of sealantmaterial. Under test conditions comparable to those encountered inaircraft operation, it has been found that the sealant compositioncontaining plastic balls is ineffective in preventing fuel leakage.

It is a principal object of this invention, therefore, to provide achannel sealant composition which is effective in preventing fuelleakage from integral fuel tanks.

Another object of the invention is to provide a channel sealant having acomposition such that loss of sealant through structural gaps throughextrusion is minimized.

A further object of the invention is to provide a sealant compositionwhich, when injected into a channel of an integral fuel tank, exerts asqueegee action in pushing out and completely replacing old sealant.

Other objects and advantages of the invention will become apparent tothose skilled in the art upon consideration of the accompanyingdisclosure and the drawing, in which:

FIG. 1 is a plan view of apparatus employed in evaluating theperformance of channel sealants, and

FIG. 2 is a cross sectional view taken along line 2--2 of FIG. 1.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention resides in the discovery thatthe addition of vulcanized rubber particles of irregular shape andhaving sharp edges and angular surfaces or faces to a non-crosslinked(uncured) elastomeric mastic provides a gap extrusion preventative thatsolves the problem of premature failure of aircraft integral fuel tanksealants. As a result of the addition of the particles, the service lifeof the sealant composition is greatly extended over that of conventionalsealants containing plastic or glass beads. The ability of theelastomeric particles to a deform at structural gaps while stilloffering resistance to extrusion prevents the loss of thenon-crosslinked mastic.

In a specific embodiment, the sealant composition of this inventioncomprises a non-crosslinked elastomeric mastic and about 10 to 100weight percent, based on the weight of the mastic, of vulcanized rubberparticles of irregular shape and having sharp edges and angular facesranging in length from about 0.010 to 0.040 inch.

In general, any of the non-crosslinked elastomers described in theliterature as being useful as channel sealants can be employed as themastic. These materials generally have the consistency of a caulkingcompound and like such compounds have the ability to be injected underpressure from a caulking gun. The materials also have other desirableproperties, e.g., maintenance of tackiness, viscosity, and adherence tometal surfaces over a wide temperature range (-65° F. to 350° F.). Afluorosilicone type of mastic, such as a 3,3,3-trifluoropropyl (methyl)siloxane polymer available from Dow Corning Corp., Midland, Mich., underthe designation DC 94011, can be utilized with advantage. A preferredmastic is poly(fluoroalkylarylenesiloxanylene) which has been found togive an outstanding sealant composition when blended with the vulcanizedrubber particles. Examples of other materials that can be used as themastic include polyurethane, poly(fluoroalkoxy phosphazene), andcyanosilicone elastomers.

The particles can be formed from any suitable vulcanizate that iscompatible with the mastic base and possesses a high strength and a highmodulus. The tensile strength of the vulcanizate should be at least 1000psi, e.g., from 1000 to 6000 psi, and it should have a hardness of atleast 80 Shore A points, e.g., 80 to 95 Shore A points. Vulcanizates offluorosilicone polymers are often preferred although vulcanizates thatgenerally meet the foregoing criteria can be satisfactorily employed.For example, particles formed from a vulcanizate prepared from the sameelastomer used as the mastic can be advantageously used.

A more comprehensive understanding of the invention can be obtained byreferring to the illustrative examples which are set forth hereinafter.In the runs described in the examples, the apparatus shown in FIGS. 1and 2 was used in testing the several sealant compositions.

As depicted in FIGS. 1 and 2, the test apparatus of metal constructionincludes a top plate member 10 having a downwardly cylindrical extension11. The combination of member 10 and extension 11 constitutes a plug 12having a cylindrical cavity 13 centrally located in its upper portion.Bottom plate member 14 is attached to ring member 16 by means ofconnecting means or connector 17. There are three connectors, only oneof which is shown in FIG. 2. Ring member 16 supporting bottom platemember 14 by means of connectors 17 is attached to top plate member 10by bolts 18 disposed around the periphery of the latter member. A VitonO-ring 19 disposed in groove 21 formed in the bottom end of plug 12provides a seal between the plug and the bottom plate member.

Positioned in a cylindrical cavity centrally located in the lowerportion of plug 12 inwardly from groove 21 is a disc or washer 22. Theupper and lower surfaces of the disc are each provided with upper andlower semicircular grooves or channels 23 and 24. Passageways 25 and 26formed in plug 12 extend from the exterior face of the plug to upperchannel 23 in disc 22. A first dam (not shown) in the upper channelseparates the points of entry of the two passageways to the groove. Asecond dam (not shown) is disposed in the upper channel opposite thefirst dam and a hole (not shown) adjacent to each side of the damcommunicates the upper channel with the lower channel.

A passageway 27 extends from the surface of cavity 13 through plug 12and disc 22. The lower end of passageway 28, which extends from thesurface of cavity 13 into plug 12, is connected to groove 21 bypassageway 29. While the gap or clearance 30 between plug 12 and bottomplate 14 can be varied, in the tests the gap was about 6 mils.

In conducting each of the tests, sealant was injected through passageway25 into upper channel 23. Using a hand operated arbor press, theinjection pressure was between 1000 and 2000 psi. Because of thepresence of the first dam, the sealant moved in a clockwise direction asit filled one-half of the upper channel. When the injected sealant metthe second dam in the upper channel, it flowed downwardly through thehole adjacent the dam into lower channel 24. As injection was continued,the sealant filled lower channel 24 in a counterclockwise direction dueto the presence of a third dam in the lower channel and then flowedthrough the second hole adjacent the dam upwardly into the unfilled halfof upper channel 23. The injection was continued until the remaininghalf of the upper channel was filled as evidenced by the extrusion ofsealant through passageway 26. Upon completion of sealant injection, theinjection port of passageway 25 and the exit port of passageway 26 weresealed with plugs. The test apparatus was then placed in a -54° C. (-65°F.) to 177° C. (350° F.) environmental chamber. A leak line wasconnected between passageway 28 and a leak collector, and a fuelpressure line was connected between passageway 27 and a pressurized fuelsource. During the test the fuel pressure line was maintained under apressure of about 20 psig with JP-4 fuel. The basic cycle consisted of 8hours at 350° F. and 16 hours at -65° F. The heat cycle included about 1hour heatup time while the cool down consumed about 1 hour out of the 16hour low temperature part of the cycle. The 24 hour heat-cool cycle wasrepeated 20 times unless failure occurred sooner as evidenced by thepresence of fuel in the fuel collector.

EXAMPLE I

A run was conducted in which vulcanized elastomer particles wereprepared for subsequent use in the preparation of the sealantcomposition of this invention. Initially, a vulcanizate was prepared ofa 3,3,3-trifluoro propyl (methyl) siloxane polymer (a product of DowCorning Corp. designated LS 422) in accordance with the followingformulation:

    ______________________________________                                                              Parts by weight                                         ______________________________________                                        3,3,3-trifluoro propyl (methyl)                                                                       100                                                   siloxane gum (LS 422)                                                         Fumed silica (Cab-o-sil MS-7)                                                                         20                                                    Fe.sub.2 O.sub.3 (Mapico Red 297)                                                                     0.8                                                   Dichlorobenzoyl peroxide (Luperco CST)                                                                1.6                                                   ______________________________________                                    

The foregoing ingredients were blended and the resulting mixture waspress cured for 5 minutes at 115° C. (240° F.) in 6"×6"×0.40" molds(1000 psi molding pressure per square inch of mold area). Post curingwas carried out in an air oven for 24 hours at 150° C. (302° F.).Physical properties of the vulcanizate are listed below.

    ______________________________________                                        Tensile strength, psi    1130                                                 Elongation %              140                                                 Hardness, Shore A points  88                                                  ______________________________________                                    

The vulcanizate was chopped to about 0.25 inch chunks by a scrap plasticchopper and then further sized by wet chopping in a close set Labconcogrinder. The particles were wet screened through a 0.0394" sieve (size18, U.S. Standard Sieve Series) and caught on a 0.0117" sieve (size 50).The particles were irregular in shape being roughly cubical with sharpangular faces. After washing with water, the particles were dried in anair circulating oven at 100° C. (212° F.).

EXAMPLE II

3,3,3-Trifluoro propyl (methyl) siloxane polymer sealant (a product ofDow Corning Corp. designated DC 94011) was blended with 20 parts byweight, based upon 100 parts by weight of the polymer sealant, of thevulcanized particles prepared as described in Example I. The sealant wasinjected into the test apparatus after which the apparatus was placed inthe environmental chamber and subjected to the test conditions describedabove. After 20 heat-cool cycles, the sealant still had not failed asevidenced by the fact that there was no fuel leakage.

EXAMPLE III

Sealant composition as prepared in Example I was used to reinject aportion of a F-111 aircraft integral fuel tank. The sealant was madeblue by blending it with 0.25 weight percent Prussian blue (ferricferrocyanide, Fe₄ [Fe(CN)₆ ]₃). The sealant was injected with a Grover223 gun with a 40 psig air supply. The blue sealant pushed out the oldwhite sealant (DC 94011) completely and was successfully reinjected pastvoids in the structure. The amount of mixing of blue and white sealantswas approximately 1" along the sealant pushed out. A plastic faced testrig was used to observe the vulcanized particles as they squeegeed thesurface clean of old sealant.

EXAMPLE IV

A sealant was prepared with poly(fluoroalkylarylene siloxanylene)[FASIL] having the following structural formula: ##STR1##

The following formulation was used in preparing the mastic:

    ______________________________________                                        Mastic                                                                                         Parts by weight                                              ______________________________________                                        Polymer (FASIL)    100                                                        Fumed silica       30                                                         Fe.sub.2 O.sub.3 (Mapico Red 297)                                                                2                                                          α,α'-bis(t-butyl peroxy)                                          diisopropyl benzene                                                                              10                                                         ______________________________________                                    

The ingredients of the formulation was dissolved in equal parts byweight of a solvent mixture of acetonitrile and acrylonitrile. Theresulting solution was heated in a closed vessel at 171° C. for 1 hourand then post treated for 24 hours in a 200° C. air oven.

The foregoing formulation was also used in preparing the vulcanizatefrom which vulcanized particles were made. The ingredients of theformulation were mixed, press molded for 20 minutes at 171° C., and postcured for 24 hours at 200° C. The vulcanizate obtained was fractured toparticles in a liquid nitrogen cooled impact grinder.

Equal parts by weight of mastic and vulcanized particles, i.e., 100weight percent particles, based on the mastic, were blended to form asealant composition. The sealant was injected into the test apparatusafter which the apparatus was placed in the environmental chamber. Thetest conditions were modified from those described above in that eachcycle consisted of 8 hours at 320° F. followed by 16 hours of cooling to77° F. During the heat-cool cycles, a fuel pressure of 20 psig wasmaintained. There was no failure after 20 cycles as shown by no fuelleakage. Failure occurred only after 4 additional cycles at 450° F. andone at -65° F.

EXAMPLE V

A commercial Poly(fluoroalkoxy phosphazene) (PNF) elastomer was reducedin viscosity by subjecting the polymer to a temperature of 204° C. (400°F.) for 4 hours in restricted contact with air. The resultant flowablepolymer was formulated as a channel sealant in accordance with thefollowing recipe:

    ______________________________________                                                              Parts by Weight                                         ______________________________________                                             Liquid PNF polymer     100                                                    Filler.sup.(1)         100                                                    Vulcanized Particles of Example I                                                                     20                                               (1)  Lithium aluminum silicate (Lathafrax)                                    ______________________________________                                    

The sealant was injected into the test apparatus and cycled under thespecified conditions, i.e., 8 hours at 350° F. and 16 hours at -65° F.while under 20 psig fuel pressure (except for week-ends). After 20 suchheat-cool cycles, the sealant had not failed. Examination of thedisassembled test apparatus showed the sealant to be capable of manymore such cycles.

EXAMPLE VI

A sealant formulated in accordance with the following recipe wasprovided by Dow Corning Corp:

    ______________________________________                                                             Parts by weight                                          ______________________________________                                        Polymer.sup.(1)        100.0                                                  Fumed silica.sup.(2)   25.2                                                   Carbon black (Ware B8064 conductive)                                                                 5.0                                                    Carbon black (Williams 1011)                                                                         1.7                                                    .sup.(1) LS/FCS 210 2:1 alternating polymer having the following              formula:                                                                       ##STR2##                                                                     .sup.(2) Cab-o-sil treated with a compound having the following formula:       ##STR3##                                                                     ______________________________________                                    

In a control run, the sealant was injected into the test apparatus andthen cycled under the conditions specified above. Failure of the sealantoccurred in one cycle.

EXAMPLE VII

The sealant prepared as described in Example VI (100 parts by weight)was blended with styrene-divinyl benzene beads (11.11 parts by weight).The amount of beads was based on the amount in commercial DC 94031sealant, a product of Dow Corning Corp. The following was the beaddistribution:

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        18-30 mesh          4.5                                                       40-70 mesh          2.0                                                       200-400 mesh        1.0                                                       ______________________________________                                    

In a control run, the sealant containing the beads was injected into thetest apparatus and then cycled under the conditions specified above.Failure of the sealant containing beads occurred in one cycle as did thebase sealant as described in Example VI.

EXAMPLE VIII

The sealant prepared as described in Example VI (100 parts by weight)was blended with vulcanized particles prepared as described in Example I(20 parts by weight). The so prepared sealant composition of thisinvention was injected into the test apparatus and then cycled under theconditions specified above. The sealant failed after completing 3cycles. Failure of the sealants in examples VI, VII and VIII resultedfrom loss of adhesion to the metal channel, allowing capillary seepageof fuel.

EXAMPLE IX

The surfaces of the test apparatus that come into contact with sealantwere cleaned to bright metal, washed with water containing detergent,and rinsed with distilled water. After being air dried, the surfaceswere primed with DC 1204 primer, a commercial hydrolyzing type primer.The sealant containing vulcanized particles as described in Example VIIIwas injected into the test apparatus and then cycled under theconditions specified above. The sealant had not failed after 20heat-cool cycles and examination indicated that the sealant would havebeen capable of many more cycles before failure.

EXAMPLE X

There is summarized in the table below the results obtained when varioussealants were injected in the test apparatus and cycled under theconditions specified above.

                  TABLE                                                           ______________________________________                                        Sealant Designation                                                                              Cycles to failure                                          ______________________________________                                        DC 94011.sup.(1)    5                                                         DC 94031.sup.(2)    5                                                         LS 77135.sup.(3)    5                                                         LS 209301.sup.(4)  11                                                         GE 651.sup.(5)     15 - Not reinjectable                                      DE 94011 plus particles.sup.(6)                                                                  20 - No failure                                            LS/FCS 210.sup.(7)  1 - Adhesion to metal failed                              LS/FCS 210 plus beads.sup.(8)                                                                     1 - Adhesion to metal failed                              LS/FCS 210 plus particles.sup.(9)                                                                 3 - Adhesion to metal failed                              LS/FCS 210 plus particles with                                                primed surfaces to test                                                       apparatus.sup.(10) 20 - No failure                                            PNF Plus particles.sup.(11)                                                                      20 - no failure                                            FASIL plus particles.sup.(12)                                                                    20 - No failure                                            ______________________________________                                         .sup.(1) Commercial sealant described in Example II.                          .sup.(2) Commerical sealant which is DC 94011 with styrene/divinyl benzen     added. The sealant is disclosed in U.S. Patent 3,580,870.                     .sup.(3) Poly(3,3,3-trifluoropropyl (methyl) siloxane) containing less        than 1 mole % methyl vinyl siloxane in the polymer and a temperature          sensitive free radical initiator.                                             .sup.(4) A variety of LS 77135.                                               .sup.(5) Commercial poly[ethylcyano(methyl)siloxane].                         .sup.(6) Sealant of this invention described in Example II.                   .sup.(7) Sealant described in Example VI.                                     .sup.(8) Sealant described in Example VII.                                    .sup.(9) Sealant of this invention described in Example VIII.                 .sup.(10) Sealant of this invention described in Example IX.                  .sup.(11) Sealant of this invention described in Example V.                   .sup.(12) Sealant of this invention described in Example IV.             

The data in the foregoing examples demonstrate that the channel sealantof this invention is effective in preventing fuel leakage underconditions comparable to those that may be encountered by aircraftintegral fuel tanks. The instant channel sealant is greatly superior toconventional sealants and provides an urgently needed solution to theproblem of premature failure of such sealants. It is noted in particularthat the channel sealant composition containing the above-describedvulcanized rubber particles has an effectiveness factor many timesgreater than that of sealants containing plastic balls as disclosed inthe patent referred to hereinabove.

As will be evident to those skilled in the art, modifications of thepresent invention can be made in view of the foregoing disclosurewithout departing from the spirit and scope of the invention.

I claim:
 1. A channel sealant composition consisting essentially of a non-crosslinked poly(fluoroalkoxy phosphazene) elastomeric mastic having the consistency of a caulking compound and an extrusion-inhibiting amount of vulcanized rubber particles of irregular shape and having sharp edges and angular surfaces, the particles being formed from a vulcanizate having a tensile strength of at least 1000 psi and a hardness of at least 80 Shore A points.
 2. The composition according to claim 1 that comprises about 10 to 100 weight percent vulcanized rubber particles, based upon the weight of the mastic.
 3. The composition according to claim 2 in which the vulcanized particles of irregular shape have sharp edges and angular faces ranging from about 0.010 to 0.040 inch.
 4. The composition according to claim 3 in which the vulcanized rubber particles are formed from a vulcanizate having a tensile strength ranging from 1000 to 6000 psi and a hardness ranging from 80 to 95 Shore A points.
 5. The composition according to claim 2 in which the particles are formed from a vulcanizate prepared from the same elastomer used as the mastic.
 6. The composition according to claim 1 in which the vulcanized rubber particles are formed from a vulcanizate of a 3,3,3-trifluoro propyl (methyl) siloxane polymer. 